Radio transmission device, radio reception device, and method for selecting transmission cancellation subcarriers

ABSTRACT

Of systematic bits (S) and parity bits (P 1 , P 2 ) generated by coding (coding rate R=1/3) transmission bits, subcarriers to which parity bits are mapped are designated as candidates for transmission cancellation and subcarriers not to be transmitted are selected from among those candidates. When this selection is made, a selection pattern which corresponds to minimum peak power of an OFDM symbol is used based on values of parity bits and phase relationship between subcarriers.

TECHNICAL FIELD

The present invention relates to a radio transmission apparatus, radioreception apparatus and method of selecting transmission cancellationsubcarriers, and more particularly, to a radio transmission apparatus,radio reception apparatus and method of selecting transmissioncancellation subcarriers in a radio communication system carrying outerror correction coding.

BACKGROUND ART

In the field of radio communication, and mobile communication inparticular, a variety of information such as image and data in additionto voice is becoming transmission targets in recent years. Since it isanticipated that the demand for transmission of various contents willincrease at an accelerated pace in the future, the necessity for morereliable and faster transmission will further increase. However, whenhigh-speed transmission is carried out in a mobile communication,influences of multipath delay signals cannot be ignored and thetransmission characteristic deteriorates due to frequency selectivefading.

As one of technologies for handling frequency selective fading, amulticarrier (MC) modulation scheme such as an OFDM (OrthogonalFrequency Division Multiplexing) scheme is becoming a focus ofattention. The multicarrier modulation scheme is a technology forrealizing high-speed transmission as a result of transmitting data usinga plurality of carriers (subcarriers) whose transmission rate issuppressed to an extent that frequency selective fading is notgenerated. Especially, because a plurality of subcarriers on which datais arranged is orthogonal to one another, the OFDM scheme is a schemewith the highest frequency utilization efficiency among multicarriermodulation schemes and it can be implemented in a relatively simplehardware configuration, and therefore the OFDM scheme is capturingspecial attention and is now under study from various angles.

As an example of such studies, there is an OFDM scheme which exercisescontrol so as to avoid transmitting subcarriers of low reception qualityin anticipation that the peak value (peak power) of transmit power willdecrease. Furthermore, in exercising this control, it tries to minimizethe deterioration of a BER (Bit Error Rate) by making bits assigned tosubcarriers not to be transmitted coincide with bits to be punctured(e.g., see “Performance of the Delay Profile Information Channel basedSubcarrier Transmit Power Control Technique for OFDM/FDD Systems”(Noriyuki MAEDA, Seiichi SAMPEI, and Norihiko MORINAGA, transactions ofInstitute of Electronics, Information and Communication Engineers, B,Vol. J84-B, No. 2, pp. 205-213 (February 2001)).

However, there is a possibility in the above described method that whenthere are subcarriers not to be transmitted, the number of bits that canbe transmitted may be decreased and the error rate characteristic maydeteriorate a great deal. Furthermore, it is necessary to transmitposition information on the subcarriers not to be transmitted from abase station to mobile stations separately, which reduces thetransmission efficiency. Moreover, simply exercising control so as toavoid transmission of subcarriers of low reception quality maycontrarily increase peak power depending on a phase relationship betweenQPSK-modulated subcarriers, etc.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a radio transmissionapparatus, radio reception apparatus and method of selectingtransmission cancellation subcarriers capable of reducing peak powerwhile suppressing deterioration of an error rate characteristic.

The present inventor has come to implement the present inventionnoticing that a parity bit is a bit with a lower degree of importancethan a systematic bit and when one bit needs to be removed, removing aparity bit has a smaller influence on the deterioration of an error ratecharacteristic than removing a systematic bit.

In order to solve the above described problem and attain the abovedescribed object, the present invention is characterized in that, ofsubcarriers to which a symbol made up of only systematic bits or onlyparity bits or a symbol made up of a mixture of both which are generatedby coding transmission bits is mapped, subcarriers not to be transmitted(that is, subcarriers whose transmission is canceled) are selected fromamong subcarriers to which a symbol made up of only parity bits ismapped. Furthermore, when subcarriers whose transmission is canceled areselected from among subcarriers to which a symbol made up of only paritybits is mapped, the present invention is characterized by selecting acombination of subcarriers which results in the lowest peak power. Thepresent invention is further characterized by not transmitting positioninformation on subcarriers whose transmission is canceled separately.With these features, the present invention allows a radio communicationsystem carrying out error correction coding to reduce peak power whilesuppressing deterioration of the error rate characteristic. The presentinvention can also prevent deterioration of transmission efficiencythrough transmission of position information.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 1 of the presentinvention;

FIG. 2 is a block diagram showing a configuration of a cancellationsection of the radio transmission apparatus according to Embodiment 1 ofthe present invention;

FIG. 3 is a block diagram showing a configuration of a radio receptionapparatus according to Embodiment 1 of the present invention;

FIG. 4 is a block diagram showing a configuration of a cancellationsection of the radio reception apparatus according to Embodiment 1 ofthe present invention;

FIG. 5 illustrates a configuration of subcarriers of an OFDM symbolaccording to Embodiment 1 of the present invention;

FIG. 6 illustrates contents of a cancellation table according toEmbodiment 1 of the present invention;

FIG. 7 illustrates subcarriers whose transmission is canceled accordingto Embodiment 1 of the present invention;

FIG. 8 illustrates reception power of subcarriers according toEmbodiment 1 of the present invention;

FIG. 9 illustrates subcarriers whose demodulation is to be excludedaccording to Embodiment 1 of the present invention;

FIG. 10 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 2 of the presentinvention;

FIG. 11 illustrates peak power according to Embodiment 2 of the presentinvention;

FIG. 12 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 3 of the presentinvention;

FIG. 13 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 4 of the presentinvention;

FIG. 14A illustrates a bit string consisting of systematic bits andparity bits according to Embodiment 4 of the present invention;

FIG. 14B illustrates a bit string consisting of systematic bits andparity bits according to Embodiment 4 of the present invention;

FIG. 14C illustrates a bit string consisting of systematic bits andparity bits according to Embodiment 4 of the present invention;

FIG. 15 illustrates subcarriers whose transmission is canceled accordingto Embodiment 4 of the present invention;

FIG. 16 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 5 of the presentinvention;

FIG. 17 is a block diagram showing a configuration of a radio receptionapparatus according to Embodiment 5 of the present invention;

FIG. 18 illustrates subcarriers whose transmission is canceled accordingto Embodiment 5 of the present invention;

FIG. 19 is a block diagram showing a configuration of a radio receptionapparatus according to Embodiment 6 of the present invention;

FIG. 20 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 6 of the presentinvention;

FIG. 21 illustrates subcarriers whose transmission is canceled accordingto Embodiment 6 of the present invention (at the time of initialtransmission);

FIG. 22 illustrates subcarriers whose transmission is canceled accordingto Embodiment 6 of the present invention (at the time of firstretransmission); and

FIG. 23 illustrates subcarriers whose transmission is canceled accordingto Embodiment 6 of the present invention (at the time of secondretransmission).

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the attached drawings, embodiments of the presentinvention will be explained in detail below.

Embodiment 1

FIG. 1 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 1 of the presentinvention. The radio transmission apparatus shown in FIG. 1 includes acoding section 12, a parallel/serial conversion (P/S) section 14, amodulation section 16, a serial/parallel conversion (S/P) section 18, aselection section 20, a cancellation table 22, a cancellation section24, an inverse fast Fourier transform (IFFT) section 26, aparallel/serial conversion (P/S) section 28, a guard interval (GI)section 30 and a transmission RF section 32, designed to transmit anOFDM symbol of a multicarrier signal in which some of a plurality ofsubcarriers making up the OFDM symbol are removed. The radiotransmission apparatus shown in FIG. 1 is mounted, for example, on abase station apparatus used for a mobile communication system.

In the radio transmission apparatus shown in FIG. 1, the coding section12 carries out error correction coding on transmission data (a bitstring) using systematic codes such as turbo codes. The coding section12 encodes a transmission bit string using systematic codes and therebygenerates systematic bits S which are transmission bits themselves andparity bits P which are redundant bits. Here, to realize a coding rateR=1/3, one systematic bit S and two parity bits P₁ and P₂ are generatedfor one transmission bit. The three bits of the systematic bit S andparity bits P₁ and P₂ are input in parallel to the P/S section 14.

The P/S section 14 converts the bit strings input in parallel to serialbit strings and inputs S, P₁ and P₂ in that order to the modulationsection 16.

The modulation section 16 BPSK-modulates the systematic bit S and paritybits P₁ and P₂ to generate a symbol. If the input bit is “0”, themodulation section 16 modulates it into a symbol of “1” and if the inputbit is “1”, the modulation section 16 modulates it into a symbol of“−1”. Because of the BPSK modulation, 1 symbol consists of 1 bit. Themodulated symbols are input to the S/P section 18 and selection section20.

Every time symbols corresponding to a plurality of subcarriersconstituting 1 OFDM symbol are input in series, the S/P section 18converts those symbols to parallel ones and inputs them to thecancellation section 24. Here, suppose the number of subcarriersconstituting 1 OFDM symbol is K=15.

Of the symbols input from the modulation section 16, the selectionsection 20 decides to which subcarriers the symbols consisting of onlyparity bits are mapped. Since the modulation section 16 in thisembodiment carries out BPSK modulation and 1 symbol consists of 1 bit,the selection section 20 decides subcarriers to which parity bits aremapped. The position of mapping to each subcarrier within 1 OFDM symbolis known for each OFDM symbol beforehand, and therefore the selectionsection 20 can easily decide subcarriers to which parity bits aremapped. For example, when the number of subcarriers constituting 1 OFDMsymbol is K=15 and coding rate R=1/3, it is known beforehand that bit Sis mapped to subcarrier f₁ , bit P₁ to f₂ , bit P₂ to f₃ , bit S to f₄ ,bit P₁ to f₅, bit P₂ to f₆, . . . , bit S to f₁₃, bit P₁ to f₁₄ and bitP₂ to f₁₅. When K=15 and R=1/3, the mapping position relationship amongS, P₁ and P₂ is the same for all OFDM symbols. When K is not divisibleby R, for example, when K=15 and R=1/4, the mapping position varies fromone OFDM symbol to another, but there is certain regularity, andtherefore the selection section 20 can easily decide subcarriers towhich parity bits are mapped in this case, too. Furthermore, even whencoded bits are punctured or interleaved, puncture patterns or interleavepatterns are known beforehand, and therefore the selection section 20can easily decide subcarriers to which parity bits are mapped based onthose patterns.

Furthermore, of L subcarriers to which parity bits are decided to bemapped, the selection section 20 selects N subcarriers (L>N) assubcarriers to be excluded from transmission (whose transmission is tobe canceled) and indicates the selected subcarriers to the cancellationsection 24. In this case, to reduce peak power of OFMD symbols, theselection section 20 references the cancellation table 22 based on thevalue of a symbol input from the modulation section 16 and selectssubcarriers whose transmission is to be canceled. The specific contentsof the cancellation table 22 and specific method of selecting subcarrierwhose transmission is to be canceled will be described later.

Here, the reason that subcarriers whose transmission is canceled are notsubcarriers to which systematic bits are mapped but subcarriers to whichparity bits are mapped is as follows. That is, when error correctioncoding is performed using systematic codes, parity bits can be said tohave a lower degree of importance than systematic bits. That is, at aradio reception apparatus which receives OFDM symbols, its error ratecharacteristic deteriorates considerably when systematic bits are lost,but a desired error rate characteristic can be maintained even if someparity bits are lost. This is attributable to the fact that systematicbits constitute transmission bits themselves, while parity bits areredundant bits.

The cancellation section 24 consists of cancellation sections 24-1 to24-K. K corresponds to the number of a plurality of subcarriers includedin 1 OFDM symbol (here K=15) and the cancellation sections 24-1 to 24-Khandle subcarriers f₁ to f_(K) respectively. The cancellation sections24-1 to 24-K each have a configuration shown in FIG. 2 and thecancellation section corresponding to a subcarrier indicated by theselection section 20 connects a switch to the B side. For example, whenthe selection section 20 selects the subcarrier f₂ as one whosetransmission is to be canceled, the cancellation section 24-2 changesthe switch from the A side to the B side. When the switch is connectedto the B side, a signal with an amplitude value “0” is input to the IFFTsection 26 for the subcarrier f₂, and therefore the IFFT section 26obtains a sample value without including subcarrier f₂. That is,transmission of the subcarrier f₂ is canceled.

The IFFT section 26 applies an inverse fast Fourier transform to symbolsor signals with amplitude values “0” input from the cancellationsections 24-1 to 24-K to transform them from a frequency area to a timearea and then inputs sample values in the time area to the P/S section28. As shown above, signals with amplitude values “0” are input from thecancellation sections corresponding to subcarriers selected by theselection section 20 and signals with symbol values “−1” or “1” areinput from the other cancellation sections, and therefore the IFFTsection 26 performs IFFT using K−N subcarriers other than thesubcarriers selected by the selection section 20. The sample valuesobtained at the IFFT section 26 are input in parallel to the P/S section28. The P/S section 28 transforms the parallel sample values after theIFFT processing into serial values. In this way, an OFDM symbol whichdoes not include subcarriers selected by the selection section 20 isgenerated.

With a guard interval added at the GI section 30, the OFDM symbol issubjected to predetermined radio processing such as up-conversion at thetransmission RF section 32 and transmitted by radio from the antenna 34.

Then, the configuration of the radio reception apparatus which receivesthe OFDM symbol transmitted from the radio transmission apparatus shownin FIG. 1 will be explained. FIG. 3 is a block diagram showing aconfiguration of the radio reception apparatus according to Embodiment 1of the present invention. The radio reception apparatus shown in FIG. 3includes an antenna 62, a reception RF section 64, a GI section 66, anS/P section 68, a fast Fourier transform (FFT) section 70, acancellation section 72, a power measuring section 74, a selectionsection 76, a P/S section 78, a demodulation section 80, an S/P section82 and a decoding section 84. The radio reception apparatus shown inFIG. 3 is mounted, for example, on a mobile station apparatus used for amobile communication system.

In the radio reception apparatus shown in FIG. 3, an OFDM symboltransmitted from the radio transmission apparatus shown in FIG. 1 isreceived by the antenna 62, subjected to predetermined radio processingsuch as down-conversion at the reception RF section 64, stripped of theguard interval at the GI section 66 and input to the S/P section 68.

The S/P section 68 serial/parallel-converts signals input in series fromthe GI section 66 into as many parallel signals as subcarriers andinputs the signals to the FFT section 70.

The FFT section 70 applies a fast Fourier transform (FFT) to the outputsignals from the S/P section 68 and transforms them from a time area toa frequency area (that is, converts the signals to symbols for therespective subcarriers) and then inputs the symbols to the cancellationsection 72 and power measuring section 74.

The power measuring section 74 measures reception power for eachsubcarrier (reception power of the respective subcarriers f₁ to f_(K))and inputs the measuring result to the selection section 76.

Of the subcarriers f₁ to f_(K), the selection section 76 selectssubcarriers to be excluded from demodulation based on the measuringresult from the power measuring section 74 and indicates the selectedsubcarriers to the cancellation sections 72. More specifically, of thesubcarriers f₁ to f_(K), the selection section 76 selects N subcarriershaving relatively small reception power. This number N is the number Nof the subcarriers selected by the radio transmission apparatus as oneswhose transmission is canceled and is a preset value. That is, the radiotransmission apparatus presets the number N of subcarriers whosetransmission is to be canceled and the selection section 76 selects Nsubcarriers from the lowest reception power as ones to be excluded fromdemodulation. This allows the radio reception apparatus to selectsubcarriers whose transmission is canceled without separatelytransmitting the position information of subcarriers whose transmissionis to be canceled from the radio transmission apparatus to the radioreception apparatus, and can thereby prevent deterioration of thetransmission efficiency caused by transmission of the positioninformation.

The cancellation section 72 consists of cancellation sections 72-1 to72-K. K corresponds to the number of a plurality of subcarriers (hereK=15) included in 1 OFDM symbol and the cancellation sections 72-1 to72-K correspond to the subcarriers f₁ to f_(K) respectively. Thecancellation sections 72-1 to 72-K each have a configuration shown inFIG. 4 and the cancellation section corresponding to a subcarrierindicated by the selection section 76 connects a switch to the B side.For example, when the selection section 76 selects the subcarrier f₂ asone to be excluded from demodulation, the cancellation section 72-2changes the switch from the A side to the B side. With the switchchanged from the A side to the B side, a signal with an amplitude value“0” is input to the demodulation section 80 through the P/S section 78for the subcarrier f₂ . In this way, demodulation of the subcarrier f₂is canceled at the demodulation section 80.

The P/S section 78 converts symbols or signals with amplitude values “0”input in parallel from the cancellation sections 72-1 to 72-K to signalsin series and inputs them to the demodulation section 80.

The demodulation section 80 BPSK-demodulates the input symbols andinputs them to the S/P section 82. If the input symbol is “1”, thedemodulation section 80 demodulates it into a bit “0” and if the inputsymbol is “−1”, the demodulation section 80 demodulates it into a bit“1”. Furthermore, for a signal with an amplitude value “0”, thedemodulation section 80 considers it as a bit “O” and inputs it to theS/P section 82. This makes it possible to obtain systematic bit S andparity bits P₁ and P₂. The parity bits whose transmission is canceled bythe radio transmission apparatus become bits “0”.

The S/P section 82 converts bits S, P₁ and P₂ input in that order toparallel bits and inputs those bits to the decoding section 84.

The decoding section 84 carries out error correction decoding such asturbo decoding using the input bits. In this way, received data (bitstring) is obtained.

Then, the operations of the radio transmission apparatus in FIG. 1 andradio reception apparatus in FIG. 3 will be explained using FIG. 5 toFIG. 9.

As shown in FIG. 5, for example, 1 OFDM symbol consists of K=15subcarriers f₁ to f₁₅ . In the case of R=1/3 as described above, it isknown beforehand that bit S is mapped to subcarrier f₁, bit P₁ to f₂ ,bit P₂ to f₃, bit S to f₄, bit P₁ to f₅, bit P₂ to f₆, . . . , bit S tof₁₃, bit P₁ to f₁₄ and bit P₂ to f₁₅. Of the subcarriers f₁ to f₁₅, theradio transmission apparatus uses the subcarriers f₂, f₃, f₅, f₆, f₈,f₉, f₁₁, f₁₂, f₁₄ and f₁₅ to which parity bits are mapped as candidatesfor transmission cancellation. When all parity bits are lost, errorcorrection coding becomes meaningless, and therefore only transmissionof some of the plurality of parity bits is canceled. Here, of L=10subcarriers to which parity bits are mapped, transmission of N=5subcarriers is canceled. This number N is a preset value. Through thistransmission cancellation, the coding rate becomes R=1/2.

The five subcarriers whose transmission is to be canceled will beselected as follows. FIG. 6 is a cancellation table showing thecorrespondence between patterns of values of bits mapped to thesubcarriers f₁ to f₁₅ (that is, patterns of values that a modulatedsymbol possibly takes) and selection patterns of subcarriers selected asones whose transmission is to be canceled. Since 1 OFDM symbol consistsof 15 subcarriers, there are a total of 2¹⁵=32768 patterns of the valuesof the bits. This table presets subcarriers whose transmission is to becanceled for patterns 1 to 32768. This setting is made based on themagnitude of peak power predicted from values of parity bits and a phaserelationship between subcarriers. That is, for the patterns 1 to 32768,selection patterns whose peak power becomes a minimum are preset fromamong ₁₀C₅ combinations of subcarriers whose transmission is to becanceled. Then, the radio transmission apparatus references thecancellation table shown in FIG. 6 based on the values of bits mapped tothe subcarriers f₁ to f₁₅ and decides subcarriers whose transmission isto be canceled. For example, when the bit value is pattern 5, iftransmission of subcarriers f₂, f₆, f₈, f₁₂ and f₁₄ out of thesubcarrier f₂, f₃, f₅, f₆, f₈, f₉, f₁₁, f₁₂, f₁₄ and f₁₅ to which paritybits are mapped is canceled, the peak power of this pattern becomes theleast among ₁₀C₅ selection patterns. When the bit value is pattern 5,the subcarriers after transmission cancellation are as shown in FIG. 7.Therefore, the radio transmission apparatus transmits an OFDM symbolconsisting of K−N=10 subcarriers f₁, f₃, f₄ , f₅, f₇, f₉, f₁₀, f₁₁, f₁₃and f₁₅ to the radio reception apparatus.

The reception power of the respective subcarriers of the OFDM symbolreceived by the radio reception apparatus is as shown in FIG. 8. Sincetransmission of the subcarriers f₂, f₆, f₈, f₁₂ and f₁₄ is canceled atthe radio transmission apparatus, their reception power becomes smallerthan that of the other subcarriers. To set N=5 subcarriers assubcarriers to be excluded from demodulation in ascending order ofreception power, the radio reception apparatus sets their amplitudevalues to “0”. As a result, the subcarriers appear as shown in FIG. 9.Thus, the radio reception apparatus obtains P₁, P₂ , P₁, P₂ and P₁ asbits “0” which are originally supposed to be mapped to the subcarriersf₂, f₆, f₈, f₁₂ and f₁₄ and transmitted.

Thus, this embodiment selects subcarriers whose transmission is to becanceled from among subcarriers to which a symbol consisting of onlyparity bits is mapped. Furthermore, a combination of subcarriers whosepeak power becomes a minimum is decided as the combination ofsubcarriers whose transmission is to be canceled. Therefore, accordingto this embodiment, it is possible to reduce peak power whilesuppressing deterioration of the error rate characteristic. Furthermore,position information of subcarriers whose transmission is canceled isnot transmitted separately, and it is therefore possible to prevent areduction of the transmission efficiency caused by transmission of theposition information.

Embodiment 2

The radio transmission apparatus according to this embodiment performstransmission cancellation only when peak power of an OFDM symbol reachesor exceeds a threshold. In other words, when peak power is lower thanthe threshold, all K=15 subcarriers are used to generate an OFDM symbolwithout transmission cancellation. Furthermore, all combination patternsof subcarriers whose transmission is to be canceled are tried and apattern corresponding to the minimum peak power is selected.

FIG. 10 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 2 of the presentinvention. In FIG. 10, the same components as those in Embodiment 1(FIG. 1) are assigned the same reference numerals and explanationsthereof will be omitted.

In the radio transmission apparatus shown in FIG. 10, a buffer 36 storessymbols input from a modulation section 16 in OFDM symbol units. Whenthe number of subcarriers constituting 1 OFDM symbol is K=15, the buffer36 stores the symbols in sets of 15 subcarriers. A peak power detectionsection 40 detects peak power of an OFDM symbol input from a P/S section28. The detected peak power value is input to a selection section 20.Furthermore, a buffer 38 stores OFDM symbols input from the P/S section28. As in the case of Embodiment 1, when transmission of N=5 subcarriersout of L=10 subcarriers to which parity bits are mapped is canceled, theselection section 20 stores ₁₀C₅ selection patterns of subcarriers whosetransmission is to be canceled.

Then, the operation of the radio transmission apparatus shown in FIG. 10will be explained. First, all switches of the cancellation sections 24-1to 24-K shown in FIG. 2 are connected to the A side. Therefore, the peakpower detection section 40 detects peak power of the OFDM symbolgenerated using all K=15 subcarriers. When the detected peak power islower than a threshold, the selection section 20 instructs the buffer 38to output this OFDM symbol. Therefore, when the peak power of the OFDMsymbol is lower than the threshold, the OFDM symbol containing nosubcarriers whose transmission is canceled is transmitted to the radioreception apparatus.

On the other hand, when the detected peak power reaches or exceeds thethreshold as shown in FIG. 11, the selection section 20 instructs thebuffer 36 to output a symbol string. The buffer 36 inputs the samesymbol string to the S/P section 18 ₁₀C₅times per 1 OFDM symbol.Furthermore, only when the detected peak power reaches or exceeds thethreshold, the selection section 20 selects N=5 of the L=10 subcarriersto which parity bits are decided to be mapped as ones whose transmissionis to be canceled and indicates the selected subcarriers to thecancellation section 24. This selection is carried out on all ₁₀C₅selection patterns. Then, every time the selection section 20 carriesout selection processing, OFDM symbols whose transmission is canceled indifferent selection patterns are stored in the buffer 38 and the peakpower is detected by the peak power detection section 40. Therefore, thebuffer 38 stores ₁₀C₅ OFDM symbols and the peak power detection section40 detects peak power Of ₁₀C₅ OFDM symbols. Then, the selection section20 selects an OFDM symbol whose peak power is a minimum out of the ₁₀C₅OFDM symbols and instructs the buffer 38 to output the selected OFDMsymbol. In this way, the OFDM symbol whose peak power is lower than thethreshold and whose peak power is a minimum is transmitted to the radioreception apparatus.

In this embodiment, instead of selecting the pattern with the minimumpower out of ₁₀C₅ selection patterns as shown above, it is also possibleto adapt the embodiment so as to detect peak power of ₁₀C₅ selectionpatterns one by one and transmit an OFDM symbol when the peak powerfalls below a threshold. By so doing, peak power may not necessarilybecome a minimum but the peak power can be made smaller than thethreshold definitely. Therefore, when peak power only needs to be lowerthan the threshold, such adaptation makes it possible to reduce theamount of processing required for transmission cancellation and areduction of peak power.

As shown above, in addition to achieving the same operations and effectsas those in Embodiment 1, this embodiment cancels transmission only whenpeak power of an OFDM symbol reaches or exceeds a threshold, and canthereby omit unnecessary transmission cancellation and consequentlyfurther suppress deterioration of the error rate characteristic whenpeak power is reduced.

Embodiment 3

A radio transmission apparatus according to this embodiment keeps thetotal transmit power of subcarriers to be transmitted constant.

FIG. 12 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 3 of the presentinvention. Note that in FIG. 12, the same components as those inEmbodiment 1 (FIG. 1) are assigned the same reference numerals andexplanations thereof will be omitted.

A selection section 20 indicates N subcarriers selected as ones whosetransmission is to be canceled to a cancellation section 24 and a powercontrol section 42.

The power control section 42 consists of power control sections 42-1 to42-K. K equals the number of a plurality of subcarriers included in 1OFDM symbol and the power control sections 42-1 to 42-K correspond tosubcarriers f₁ to f_(K) respectively. The power control section 42assigns transmit power corresponding to subcarriers whose transmissionis canceled to subcarriers whose transmission is not canceled. That is,the transmit power which decreases because transmission of subcarriersselected by the selection section 20 is canceled is assigned tosubcarriers other than subcarriers whose transmission is to be canceled.This assignment is performed more specifically as follows.

When the selection section 20 selects N out of K subcarriers included in1 OFDM symbol as ones whose transmission is to be canceled, the powercontrol sections corresponding to the N subcarriers indicated by theselection section 20 out of the power control sections 42-1 to 42-Kmultiply the transmit power of K−N subcarriers (that is, subcarrierswhich are transmitted) other than subcarriers whose transmission is tobe canceled by K/(K−N) respectively. For example, when K=15 and N=5, thetransmit power of N=5 subcarriers is multiplied by 1.5 compared to thecase where no transmission cancellation is performed. By so doing, it ispossible to equally assign transmit power corresponding to the transmitpower decrease due to cancellation of transmission of the subcarriers,to subcarriers other than the subcarriers whose transmission is to becanceled.

Thus, this embodiment assigns transmit power corresponding to thetransmit power decrease because of cancellation of transmission of thesubcarriers to subcarriers other than the subcarriers whose transmissionis to be canceled, and can thereby reduce peak power while keepingtransmit power of OFDM symbols constant.

Embodiment 4

This embodiment will describe a case where a modulation section 16modulates two or more bits into 1 symbol.

FIG. 13 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 4 of the presentinvention. Note that the same components in FIG. 13 as those inEmbodiment 1 (FIG. 1) are assigned the same reference numerals andexplanations thereof will be omitted.

The modulation section 16 performs QPSK modulation on bits input from aninput order control section 46. That is, the modulation section 16generates 1 symbol for every 2 bits which are input successively.

A P/S section 14 outputs bits S, P₁, P₂ in that order as shown in FIG.14A. Thus, the coding rate is R=1/3 at this time point.

Here, suppose, for example, that a puncture section 44 performspuncturing to change the coding rate to R=1/2. In this case, thepuncture section 44 punctures parity bits. To set the coding rate toR=1/2, it is necessary to make the puncture section 44 output 1 paritybit per 1 systematic bit. Therefore, the puncture section 44 puncturesP₁ and P₂ alternately. As a result, the bit string output from thepuncture section 44 is as shown in FIG. 14B. This bit string is input tothe input order control section 46.

Here, the modulation section 16 generates 1 symbol for every 2 bitsinput successively (performs QPSK modulation), and therefore if the bitstring in FIG. 14B is input to the modulation section 16 in its originalorder, no symbol consisting of only parity bits is generated, whichmakes it impossible to select subcarriers whose transmission is to becanceled.

Therefore, the input order control section 46 rearranges the bit stringshown in FIG. 14B as the bit string shown in FIG. 14C. That is, theinput order control section 46 controls the order in which thesystematic bits and parity bits input from the puncture section 44 areinput to the modulation section 16. More specifically, the input ordercontrol section 46 performs control in such a way that two parity bitsare input successively to the modulation section 16. In this way, in themodulation section 16, symbols consisting of only parity bits aregenerated.

When rearranged as shown in FIG. 14C, symbols consisting of S and S andsymbols consisting of P₂ and P₁ are generated and the respective symbolsare mapped to the subcarriers f₁ to f₁₅. Of the subcarriers to whichthese symbols are mapped, the selection section 20 selects subcarriersto which symbols consisting of P₂ and P₁, that is, symbols consisting ofonly parity bits are mapped as candidates for transmission cancellation(FIG. 15). Then, the selection section 20 cancels transmission of onlysome of these candidates. In FIG. 15, of the subcarriers f₂, f₄, f₆, f₈,f₁₀, f₁₂ and f₁₄ to which symbols consisting of P₂ and P₁ are mapped,transmission of the subcarriers f₄, f₁₀ and f₁₂ is canceled. This causesthe coding rate to be R=2/3.

This embodiment has explained QPSK modulation as an example, but thisembodiment is also applicable to modulation schemes whereby three ormore bits are modulated into one symbol (8 PSK, 16 QAM, 64 QAM, etc.).For example, in the case where the modulation scheme is 16 QAM, theinput order control section 46 performs control in such a way that fourparity bits are input successively to the modulation section 16.

As shown above, even when the modulation section modulates two or morebits into 1 symbol, this embodiment can definitely generate subcarrierscarrying only parity bits and select subcarriers whose transmission isto be canceled.

Embodiment 5

A radio transmission apparatus according to this embodiment selectssubcarriers whose reception power at a radio reception apparatus fallsto or below a threshold out of subcarriers to which symbols consistingof only parity bits are mapped as subcarriers whose transmission is tobe canceled.

FIG. 16 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 5 of the presentinvention. Note that the same components in FIG. 16 as those inEmbodiment 1 (FIG. 1) are assigned the same reference numerals andexplanations thereof will be omitted. Furthermore, FIG. 17 is a blockdiagram showing a configuration of a radio reception apparatus accordingto Embodiment 5 of the present invention. Note that the same componentsin FIG. 17 as those in Embodiment 1 (FIG. 3) are assigned the samereference numerals and explanations thereof will be omitted.

In the radio transmission apparatus shown in FIG. 16, pilot signals aremodulated by a modulation section 16, passed through an S/P section 18and a cancellation section 24, and mapped to subcarriers f₁ to f₁₅ whichconstitute 1 OFDM symbol. Then, an OFDM symbol consisting of pilotsignals is transmitted to the radio reception apparatus shown in FIG.17.

In the radio reception apparatus shown in FIG. 17, a power measuringsection 74 measures reception power of the subcarriers f₁ to f₁₅ of theOFDM symbol consisting of pilot signals. Then, the power measuringsection 74 inputs notification information for notifying the radiotransmission apparatus of a reception power value of each subcarrier toa modulation section 86. This notification information is modulated bythe modulation section 86, up-converted by a transmission RF section 88and transmitted from the antenna 62 to the radio transmission apparatus.

In the radio transmission apparatus shown in FIG. 16, notificationinformation received through an antenna 34 is down-converted by areception RF section 48 and demodulated by a demodulation section 50.The demodulated notification information is input to a selection section20. The selection section 20 compares reception power values of thesubcarriers f₁ to f₁₅ with a threshold and selects subcarriers whosereception power values are equal to or lower than the threshold out ofsubcarriers to which symbols consisting of only parity bits are mappedas subcarriers whose transmission is to be canceled.

For example, as shown in FIG. 18, when reception power of subcarriersf₅, f₉, f₁₁ and f₁₂ out of subcarriers f₂ , f₃ , f₅ , f₆ , f₈, f₉ , f₁₁,f₁₂, f₁₄ and f₁₅ to which parity bits P₁ and P₂ are mapped falls to orbelow a threshold, the selection section 20 selects these foursubcarriers as subcarriers whose transmission is to be canceled.

Thus, this embodiment does not transmit subcarriers whose receptionpower at the radio reception apparatus falls to or below a threshold outof subcarriers to which symbols consisting of only parity bits aremapped, and can thereby prevent unnecessary transmission of parity bitswhich are expected not to be received correctly at the radio receptionapparatus.

Embodiment 6

ARQ, and H-ARQ in particular, is a technology for improving an errorrate by combining received signal (symbol) for every time retransmissionis performed. In order to improve an error rate, the H-ARQ requires aradio reception apparatus to combine received signals. However, whenthere are subcarriers whose transmission is to be canceled, symbolsmapped to those subcarriers are not transmitted and if transmission ofthe same subcarriers as those at the time of initial transmission isalso canceled at the time of retransmission, the symbols mapped to thesubcarriers are not transmitted at the time of retransmission either.This means that there exist symbols that cannot be combined at the radioreception apparatus and the error rate will not improve at all no matterhow many times retransmission may be performed. Therefore, the radiotransmission apparatus according to this embodiment selects differentsubcarriers between the time of initial transmission and the time ofretransmission from among the subcarriers to which symbols consisting ofonly parity bits are mapped as the subcarriers whose transmission is tobe canceled in a communication system which carries out H-ARQ (HybridAutomatic Repeat reQuest).

FIG. 19 is a block diagram showing a configuration of a radio receptionapparatus according to Embodiment 6 of the present invention. In FIG.19, the same components as those in Embodiment 1 (FIG. 3) are assignedthe same reference numerals and explanations thereof will be omitted.Furthermore, FIG. 20 is a block diagram showing a configuration of aradio transmission apparatus according to Embodiment 6 of the presentinvention. In FIG. 20, the same components as those in Embodiment 1(FIG. 1) are assigned the same reference numerals and explanationsthereof will be omitted.

In the radio reception apparatus shown in FIG. 19, a decoding result(bit string) obtained by a decoding section 84 is input to an errordetection section 90. The error detection section 90 carries out errordetection such as CRC (Cyclic Redundancy Check) on the input decodingresult. Then, the error detection section 90 generates an ACK(ACKnowledgment: positive response) or NACK (Negative ACKnowledgment:negative response) based on the error detection result and inputs it toa transmission RF section 92. The error detection section 90 generatesan ACK when the decoding result is OK with no error or generates a NACKwhen the decoding result is NG with some error as a response signal tothe error detection and inputs it to a transmission section 92. Thetransmission section 92 transmits ACK/NACK to the radio transmissionapparatus shown in FIG. 20 through an antenna 62.

At the radio transmission apparatus shown in FIG. 20, a signal includingthe ACK or NACK transmitted from the radio reception apparatus shown inFIG. 19 is received by an antenna 34, subjected to predetermined radioprocessing such as down-conversion at the reception RF section 52 andinput to an ACK/NACK detection section 54. The ACK/NACK detectionsection 54 detects the ACK or NACK from the input signal and inputs itto a retransmission control section 56. Symbols generated by amodulation section 16 are input to the retransmission control section56. The retransmission control section 56 stores symbols input from themodulation section 16 and at the same time inputs the symbols to an S/Psection 18 and a selection section 20. Then, when a NACK is input fromthe ACK/NACK detection section 54, the retransmission control section 56retransmits a symbol corresponding to the NACK. The retransmitted symbolis also input to the S/P section 18 and selection section 20.

At the time of initial transmission, the selection section 20 performsthe same operation as that in Embodiment 1 and stores the selectionresult in a selection result storage section 58. Then, at the time offirst retransmission, the selection section 20 references the selectionresult at the time of initial transmission stored in the selectionresult storage section 58 and selects subcarriers different from thesubcarriers at the time of initial transmission as subcarriers to beexcluded from transmission. This selection result is also stored in theselection result storage section 58. Furthermore, at the time of secondretransmission, the selection section 20 references the selection resultat the time of initial transmission and the selection result at the timeof first retransmission stored in the selection result storage section58 and selects subcarriers different from the subcarriers at the time ofinitial transmission and at the time of first retransmission assubcarriers to be excluded from transmission. That is, the subcarriersselected by the selection section 20 at the time of retransmission assubcarriers whose transmission is to be canceled are selected from amongsubcarriers other than the already selected subcarriers. In other words,subcarriers selected at the time of retransmission as subcarriers whosetransmission is to be canceled are selected only from among thesubcarriers already transmitted before the time of retransmission. Thiswill be explained more specifically using FIG. 21 to FIG. 23 below. FIG.21 shows a case at the initial transmission, FIG. 22 shows a case at thefirst retransmission and FIG. 23 shows a case at the secondretransmission.

In FIG. 21 to FIG. 23, of subcarriers f₁ to f₁₅, subcarriers f₂, f₃, f₅,f₆, f₈ , f₉, f₁₁, f₁₂, f₁₄ and f₁₅ to which parity bits are mapped aredesignated as candidates for transmission cancellation and of these L=10candidates for transmission cancellation, transmission of N=3subcarriers is canceled. The three subcarriers whose transmission is tobe canceled are selected as follows. That is, when f₂, f₆ and f₈ areselected at the time of the initial transmission as subcarriers whosetransmission is to be canceled (FIG. 21) if f₂ , f₆ and f₈ are selectedagain at the time of the first retransmission, this means that the samesubcarriers are not transmitted again. Therefore, at the time of thefirst retransmission, transmission of subcarriers f₃, f₁₁ and f₁₄ whichare different from those at the time of the initial transmission iscanceled (FIG. 22). All these subcarriers f₃, f₁₁ and f₁₄ are thesubcarriers already transmitted at the time of the initial transmission.Furthermore, at the time of the second retransmission, transmission ofsubcarriers f₅ , f₉ and f₁₂ which are different from those at the timeof the initial transmission and at the time of the first retransmissionis canceled (FIG. 23). Thus, the selection section 20 selects differentsubcarriers whose transmission is to be canceled between the time of theinitial transmission and the time of retransmission, and, at the time ofretransmission, selects the subcarriers whose transmission is to becanceled only from among the subcarriers already transmitted at the timeof the initial transmission. Furthermore, when retransmission is carriedout over a plurality of times, as long as there remain candidates fortransmission cancellation whose transmission is not canceled yet,subcarriers whose transmission is to be canceled are selected from amongthe subcarriers whose transmission is not canceled yet.

Thus, this embodiment selects different subcarriers as subcarriers whosetransmission is to be canceled between the time of the initialtransmission and the time of retransmission, and, at the time ofretransmission, selects the subcarriers whose transmission is to becanceled only from among the subcarriers already transmitted at the timeof the initial transmission, and thereby preventing occurrence ofsubcarriers that are not transmitted even upon retransmission andreliably improving the error rate characteristics upon everyretransmission.

As described above, the present invention allows a radio communicationsystem which carries out error correction coding to suppressdeterioration of its error rate characteristic and at the same timereduce peak power.

This application is based on the Japanese Patent Application No.2002-266396 filed on Sep. 12, 2002, entire content of which is expresslyincorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention is preferably applicable to a radio communicationterminal apparatus and radio communication base station apparatus, etc.,used for a mobile communication system.

1. A radio transmission apparatus comprising: a coding section thatencodes a transmission bit to generate a systematic bit and a paritybit; a modulation section that modulates the systematic bit and theparity bit generated by said coding section to generate a symbol; aselection section that carries out selection processing of selecting asubcarrier to which a symbol consisting of only parity bit is mapped outof the symbols generated by said modulation section as a subcarrier tobe excluded from transmission; a generating section that generates amulticarrier signal using subcarriers other than the subcarrier selectedby said selection section out of subcarriers to which the symbolsgenerated by said modulation section are mapped; and a transmissionsection that transmits the multicarrier signal generated by saidgenerating section.
 2. The radio transmission apparatus according toclaim 1, further comprising a table showing the correspondence between aplurality of patterns of values that are possibly taken by the symbolsgenerated by said modulation section and a plurality of selectionpatterns of subcarriers selected by said selection section assubcarriers to be excluded from transmission, wherein said selectionsection references said table based on the values of the symbolsgenerated by said modulation section and selects a selection patternwith the minimum peak power out of said plurality of selection patterns.3. The radio transmission apparatus according to claim 1, furthercomprising a detection section that detects peak power of themulticarrier signal generated by said generating section, wherein saidselection section selects a selection pattern with the minimum peakpower out of a plurality of selection patterns of the subcarriers to beexcluded from transmission.
 4. The radio transmission apparatusaccording to claim 1, further comprising a detection section thatdetects peak power of the multicarrier signal generated by saidgenerating section, wherein said selection section carries out saidselection processing only when the peak power reaches or exceeds athreshold.
 5. The radio transmission apparatus according to claim 1,further comprising a detection section that detects peak power of themulticarrier signal generated by said generating section, wherein saidtransmission section transmits a multicarrier signal whose peak power islower than a threshold out of the multicarrier signals generated by saidgenerating section.
 6. The radio transmission apparatus according toclaim 1, further comprising a power control section that assigns powercorresponding a power decrease that occurs when transmission of thesubcarrier selected by said selection section is excluded to asubcarrier other than the subcarrier to be excluded from transmission.7. The radio transmission apparatus according to claim 6, wherein saidselection section selects N subcarriers of K subcarriers to which thesymbols generated by said modulation section are mapped as subcarriersto be excluded from transmission, and said power control sectioncontrols power of K−N subcarrier other than subcarriers to be excludedfrom transmission to the power multiplied by K/(K−N).
 8. The radiotransmission apparatus according to claim 1, wherein said modulationsection generates one symbol for every two or more bits inputsuccessively, said radio transmission apparatus further comprising anorder control section that controls the order in which the systematicbit and the parity bit generated by said coding section are input tosaid modulation section so that a symbol consisting of only the paritybit is generated.
 9. The radio transmission apparatus according to claim1, wherein said selection section selects, as the subcarrier to beexcluded from transmission, a subcarrier to which a symbol consisting ofonly the parity bit out of the symbols generated by said modulationsection is mapped and for which a pilot whose reception power measuredby a radio reception apparatus that receives the multicarrier signalfalls to or below a threshold.
 10. The radio transmission apparatusaccording to claim 1, further comprising a retransmission section thatretransmits the symbol generated by said modulation section, wherein atthe time of retransmission by said retransmission section, saidselection section selects the subcarrier different from the subcarrierat the time of initial transmission.
 11. A radio reception apparatuscomprising: a reception section that receives the multicarrier signaltransmitted from the radio transmission apparatus according to claim 1;a measuring section that measures reception power of a plurality ofsubcarriers constituting the multicarrier signal received by saidreception section; and a selection section that selects subcarriers tobe excluded from demodulation, said subcarriers to be excluded fromdemodulation selected up to a number of the subcarriers regarded assubcarriers to be excluded from transmission by said radio transmissionapparatus from a subcarrier corresponding to a lowest reception powermeasured by said measuring section.
 12. A method of selectingtransmission cancellation subcarrier comprising, where symbolsconsisting of one or both of a systematic bit and a parity bit generatedby coding a transmission bit are mapped to subcarriers, selecting asubcarrier whose transmission is to be canceled from among thesubcarriers to which the symbols consisting of only the parity bit aremapped.